Hydraulic supply system for an aeronautical propeller or the like



July 15, 1958 R P. LAMBECK ET AL 2,843,140

HYDRAULIC SUPPLY SYSTEM FOR AN AERONAUTICAL PROPELLER OR THE LIKE Filed April 3, 1956 IN VEN TORS PAY/"0ND R t AMBECK AMT/ION) V- Pad/D BY M f- WM United States Patent HYDRAULIC SUPPLY SYSTEM FOR AN AERO- NAUTICAL PROPELLER OR THE LIKE Raymond P. Lambeck and Anthony V. Pond, Manchester, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application April 3, 1956, Serial No. 575,761

7 Claims. (Cl. 137-110) This invention relates to an hydraulic system and, more specifically, to an improved supply system for an hydraulically operated adjustable pitch aeronautical propeller. While not limited to any specific use or application, the system of this invention may be used to particular advantage in supply ing aeronautical propeller installations wherein engine oil is used as the operating fluid.

It is the general object of the invention to provide a system which will continue to supply hydraulic fluid even in the event the main supply line of the system is broken or damaged or for some other reason loses pressure.

A more specific object of the invention is to provide a system which will not only continue to supply fluid to an hydraulically operated device in the event the main supply line is broken, but which will operate automatically to shut off the main supply line so as to minimize the 1 fluid loss and thereby diminish the fire hazard and other dangers which accompany hydraulic fluid leakage.

Other, more specific objects and advantages of the invention will become apparent to those skilled in the art from the following description taken in connection with the attached drawing wherein the single view shows the hydraulic supply system schematically. It will be understood that the drawing and description relating to but one embodiment of the invention are not to be taken as limiting or defining the scope of the invention, the claims forming a part of this specification being relied uponfor that purpose.

There are many aeronautical propeller installations which use engine oil as the operating fluid to effect pitch changes including the major changes in propeller pitch such as occur when the propeller is feathered and unfeathered, reversed and then turned back to the positive pitch range. The propeller governor or other hydraulically operated propeller control mechanism is generally located on the nose section of the engine, but the pump or pumps used to supply oil to the said mechanism may be located in a rear section of the engine near an engine oil tank which provides the fluid source for hydraulic operation. 1

In order to describe the invention in an exemplary environment, the improved hydraulic supply system will be described as particularly adapted to supply engine oil to a propeller governor or control mechanism which is particularly adapted to feather and unfeather the blades of an aeronautical propeller. It will be understood, however, that the system asdescribed herein can be employed to supply a propeller control mechanism with hydraulic fluid for additional or other purposes and it will also be understood that the system described herein can be employed to advantage as the hydraulic supply system for apparatus other than an aeronautical propeller.

A conventional system for supplying a propeller with hydraulic fluid for feathering and unfeathering operations may 'include, as'a pressurized fluid source, a pump which when operated will withdraw oil from an engine tank adjacent the rear of the engine and the system may also include an elongated supply line or conduit extending from the said pump to the propeller governor or control mechanism located in the nose section of the engine. The improved system of this invention utilizes a conventional pump 10 which may be located at the rear section of an engine to withdraw oil from an engine tank for the purpose of supplying the same under pressure to a conventional propeller governor or control mechanism indicated schematically at 12. The improved system also includes an elongated main supply conduit or other passage defining means 14, but in addition thereto, the system includes anauxiliary or emergency oil line or conduit 16. In addition to the foregoing, the system of this invention includes certain flow control valves and passages, all of which are enclosed within the broken line rectangle A for purposes of illustration, and all of which can conveniently be located adjacent the rear end or in a rearwardly spaced section of the engine. Additionally, the system of this invention includes a pair of check valves and additional conduits, which are enclosed within the broken line rectangle B for illustration, and all of which can conveniently be located adjacent the propeller in the nose section of the engine installation An important element included among the control devices located rearwardly in the engine installation is a two position selector valve indicated generally by the reference numeral 18. The valve 18 comprises an elongated body or stem 20 which is reciprocable axially and which has two spaced apart valve lands 22 and 24, the lands being adapted to close outlet ports 26 and 28, respectively, in a valve chamber 30 defined within a valve housing or other suitable means. The chamber 30 is connected with a conduit 32 receiving the oil or other hydraulic fluid under pressure from the pump 10. The valve lands 22 and 24 are axially spaced on the valve body or stem 20 so that when the land 24 closes the outlet 28 in one position of the valve, the outlet 26 will be opened, and when the land 22 closes the outlet 26 in another position of the valve, the outlet 28 will be opened. As will be described in greater detail hereinafter, the outlet 26 communicates with the main supply line or conduit 14 and the outlet 28 communicates with the auxiliary or emergency supply line 16.

A piston 34 is provided on the end of the stem 20 of the valve 18 beyond the valve land 22, the said piston being reciprocable within a chamber 36 defined within the valve housing or other means. A spring 38 is disposed in the chamber 36 and seated upon the piston 34 to bias the valve 18 in one axial direction toward the aforementioned one position whereby the land 22 is biased to open position with respect to the outlet 26 and the land 24 is biased to closed position with respect to the outlet 28. In addition to the force of the spring 38, the valve 18 may be biased by fluid under pressure introduced to the chamber 36 from a conduit 40 as will be described hereinafter.

A piston 42 is provided on the opposite end of the body or stem 20 of the valve 18 and is reciprocable within a chamber 44 defined within the aforesaid housing or other means. There is no spring provided within the chamber 44 to bias the selector valve 18 oppositely to the previously described biasing force, but fluid under pressure may be introduced to the chamber 44 from a conduit 46 for that purpose as will be described hereinafter.

In the biased position of the selector valve 18 as shown in the drawing, the oil or other hydraulic fluid introduced to the valve chamber 30 at pump pressure from the pump 10 will flow through the outlet 26 to a passage or conduit 48 communicating with a pressurizing valve indicated generally at 50 and which is disposed between the conduit a ball check valve 52 in the end of the conduit 48 is closed when fluid is introduced under pressure to the said conduit. The pressurizing valve 58 includes a valve member or body 54 which is reciprocable within a chamber 56. The chamber 56 has an inlet 58 communicating with the conduit 48 and an outlet which communicates with the main supply line 14. The said chamber also has a drain communicating with the conduit 68 which will'receive any fluid leaking around either end of the valve body 54 and return the same to the tank or other source communicating with the pump 10. A spring 62 is disposed in the chamber 56 to bias the valve member 54 into the position shown wherein communication is closed between the chamber inlet 58 and the main supply line 14.

When hydraulic fluid introduced through the inlet 58 to the valve chamber 56 reaches a preselected pressure, as may be determined by selection of a spring 62 having a desired force, the valve member 54 is moved within the chamber 56, the hydraulic pressure having overcome the force of the said spring. In the moved or adjusted position of the valve member 54, which will be apparent from the drawing, the hydraulic fluid will flow through the chamber 56 from the inlet 58 to the main supply line 14.

It will be observed that the conduit 40 communicates with the main line 14 whereby fluid at system pressure will be introduced to the piston chamber 36 to supplement the force of the spring 38 therein in biasing the selector valve 18 to the position shown wherein fluid flow is directed to the main line. It will also be observed that a passage 66 is provided between the conduit 40 and the conduit 48, the said passage being closed by the ball check 52 when the conduit 48 contains fluid under pressure. The passage 66 will only be opened under conditions which will be described hereinafter.

The hydraulic fluid under pressure will flow in the main supply line 14 from the rear section of the engine forwardly to the nose section of the engine and on to the propeller control mechanism 12 after it has passed a check valve indicated by the reference numeral 68. As will be apparent from the drawing, the check valve 68 is reciprocable in a chamber 70 defined in a housing or other means and is biased by a spring 72 disposed in said chamber to a position wherein communication is closed between the main supply line 14 and the inlet conduit 74 for the propeller control mechanism 12. Communication is effected between the main supply line 14 and the propeller inlet conduit 74 by displacing the valve 68 against the force of the spring 72. The pressure of the hydraulic fluid in the main line 14 is the force causing displacement of the valve 68, and such displacement is sufficient to provide an opening between the main line 14 and a valve by-pass conduit or passage 76 which communicates with the propeller inlet conduit 74. The said inlet conduit 74 also communicates with the chamber 70 whereby the fluid pressure at the said inlet will also oppose the aforedescribed displacement of the valve 68. However, the spring 72 is a low force spring and the valve 68 will open when the pressure in line 14 is only slightly higher than the pressure in passage 76 and chamber 70.

It will be observed that a second check valve similar to the valve 68 is disposed in the emergency or auxiliary line 16 between that conduit and the propeller inlet conduit 74. The construction and arrangement of the last mentioned check valve is substantially identical with the valve 68 and for that reason no additional description need be made. For convenience, the second check valve is designated by the reference 68a, the valve chamber by 70a, the valve spring by 72a, and the by-pass passage by 76a so that they may be distinguished from theiridentical counterparts in the first mentioned check valve.

It should be noted that the propeller inlet conduit 74 communicates with the valve chamber 70a so as to provide hydraulic pressure assisting the spring 72a to maintain the valve 68a in closed position. However, the

aforesaid hydraulic pressure and spring force on the valve 68a are opposed by pressure of hydraulic fluid in the auxiliary line 16. That is, the auxiliary or emergency line 16 will receive hydraulic fluid from the pump 10 through a bleed passage 78 extending from a conduit 80 communicating with the pump discharge conduit 32. The bleed passage 78 communicates with the emergency line 16 at the selector valve 18 but between the land 24 and the piston 42 on one end thereof. Obviously, the flow rate through the bleed passage 78 is less than the flow rate through the selector valve to the conduit 14 or to the conduit 16. And, obviously, more time will be taken in creating a pressure within the auxiliary line 16 than is taken in creating the same pressure at the pressurizing valve 50 and in the main conduit 14. However, during normal operation when flow to the propeller mechanism 12 is through the main line 14 and check valve 68, the pressure in the auxiliary line 16 will substantially equal the pressure in the main line 14. This pressure in the auxiliary line 16 will be reflected in the chamber 44 at the end of the piston 42 and will also be reflected in a passage 82 which communicates with the branch passage 80 but which is normally closed by a ball check 84 responsive to pressure in said branch passage. Having created an hydraulic pressure in the auxiliary conduit 16 and in the piston chamber 44 which is substantially equal to the pressure in the main line 14 during flow therethrough, it will be readily seen that the selector valve 18 is retained in the position shown only by the force of the valve spring 38.

It will also be quite apparent that the pressure in the propeller inlet 74 will be slightly lower than the pressure in the pump discharge line 32 due to the normal flow losses in the conduits 48 and 14 and the pressure drop across the check valve 68 and also across the valve 50 when the valve body 54 is only partially open. This pressure differential between the conduits 32 and 74 will cause some flow to take place through the bleed passage 78, the auxiliary line 16 and through a bleed passage (to be described) in the check valve 68a. If this flow through the auxiliary line 16 is greater than can be accommodated by the bleed in valve 68a, the said valve will open slightly to pass the excess into passage 76a.

When the propeller is not being feathered or unfeathcred, the pump 10 is de-energized and the hydraulic pressure within the system is dissipated. Obviously, there will be extended periods of non-use of the hydraulic supply system and, therefore, it is desirable to have some oil flow through the system to prevent congealing of oil in the conduits. Also, under low temperature conditions, some flow is desirable to prevent freezing. Bleed flow to prevent congealing and possible freezing is accomplished by the flow of warm oil from the propeller governing mechanism 12 through the system to the pump 10. The oil for such bleed flow can be provided from the source used to operate the propeller mechanism during the continuous and conventional pitch change. Flow through the check valve 68 from the propeller to the pump is accommodated by means of a bleed valve 86 disposed in the valve 63 and biased by a spring 88 to the position shown. It will be observed that the bleed valve 86 has a central passage 90 providing communication between the main line 14 and the propeller inlet conduit 74. The bleed valve 86 is closed to prevent such bleed flow only when hydraulic pressure in the propeller inlet conduit 74 so far exceeds pressure in the main line 14 as to cause the bleed valve 86 to close against the force of the spring 88. This condition occurs only when there is a break in the main line 14 as will be hereinafter described. The bleed flow in the main line 14 reaches the pump 10 through the passage 66 and the pressurizing conduit 48, the ball check 52 being open when the pump 10 is not in operation.

A similar bleed valve identified by reference 86a is provided in the check valve 68a, the said bleed valve being biased to open position by a spring 88a and having a bleed passage 90a. The bleed valve 86a will permit flow from the propeller inlet conduit 74 into the auxiliary line 16 and on to the pump through the passage 82, the ball check 84 being open when the pump 10 is not operatmg.

In order fully to understand the advantages of the hydraulic system provided in accordance with the present invention, assume that a break occurs in the main line 14 during operation of the pump 10 when flow should be accommodated through the said main line. Obviously, a break in the main line 14 will so reduce the pressure therein and in the conduit 40 and the spring chamber 36 as to cause the selector valve 18 to be biased to the position wherein the land 22 closes the valve port 26 and the land 24 is displaced from the valve port 28. The force biasing the selector valve 18 against the force of the spring 38 to the adjusted position described is provided by the hydraulic pressure in the auxiliary line 16 and in the piston chamber 44. In this connection, it should be observed that the valve spring 38 is selected to have a biasing force substantially less than the biasing force provided by the fluid in the chamber 44 under normal operating conditions. When the selector valve is positioned to open the valve port 28, oil flows directly from the pump discharge conduit 32 to the emergency line 16 through the said open port. Sustained hydraulic flow in the emergency line 16 at pump pressure opens the check valve 68a and retains the same in the opened position so that the hydraulic supply to the propeller mechanism 12 is continued with substantially no interruption.

When the main supply line 14 is broken as mentioned above, the check valve 68 will be closed by the spring 72. Furthermore, the hydraulic pressure in the propeller inlet conduit 74 and within the check valve chamber 70 will be unopposed by hydraulic pressure in the main line 14 and, as a result, the bleed valve 88 will be pressure biased to closed position. Accordingly, there can be no flow through the check valve 68 into the broken main line 14 and since the land 22 on the selector valve 18 closes the valve port 26, there can be no fluid pumped into-the broken main line 14. Thus, the system will continue to operate despite a break in the main line.

It should at this point be observed that a break may occur in the main line 14 during a period of non-use and while there is no pressure in the line 16 and chamber 44 to position the valve 18 against the force of the spring 38. If this should occur and then pump operation is started, fluid will be pumped through the selector valve port 26 to the pressurizing valve 50 'which'will create a back pressure in the pump discharge conduit 32 and the conduit 80 to cause flow through the bleed passage 78 into the emergency conduit 16 and valve piston chamber 44. Thus, pressure will build up in the chamber 44 to provide the biasing force necessary to position the valve in the aforementioned other position for flow through the valve port 28 and the emergency conduit 16.

If duringnormal operation of the hydraulic supply system with the valve port 26 open and the valve port 28 closed and with flow occurring through the main line 14 a break occurs in the auxiliary line 16, the check valve 68a will be closed to prevent flow through the said check valve into the broken emergency line. However, .there will be limited flow into the broken emergency line 16 from the pump 10. That is, there will be some flow through the bleed passage 78 to the emergency line 16, this limited flow resulting in some leakage.

In the event the selector valve 18 should be in the other position wherein the port 28 is open and the port 26 is closed while both the main line and the auxiliary lines areundamaged, subsequent operation of the pump 18 will cause flow through the emergency line 16 and no flow will take place through the main line 14. Then, if the emergency line 16 is broken, pressure in the piston chamber 44 will be dissipated so that the spring 38 will bias the valve 18 'to the position shown wherein the valve port 28 is closed-and the valve port 26 is opened. Hydraulic pressure will then build in the pressurizing conduit 48 until the pressurizing valve 50 opens to commence flow through the main line 14 to the propeller control mechanism 12. In this circumstance, and as previously described, the check valve 68a will be closed and the bleed valve 86a will be closed.

While the invention has been described with specific reference to the embodiment shown and with detailed reference to the valve constructions, it will be readily understood that changes can be made in the details of valve construction and that other modifications can be made without departing from the scope of the invention. Therefore, it is not the intent to limit the claims to the specific embodiment shown and described otherwise than indicated by the claims which follow.

The invention claimed is:

1. In a system for supplying fluid from a pressurized source, the combination comprising means defining two fluid conduits, a selector valve disposed between said conduits and said source and having one position wherein one of said conduits is connected with said source and another position wherein the other of said conduits is connected with said source, said valve being biased toward said one position by fluid under pressure in said one conduit and biased toward said other position by fluid under pressure in said other conduit, resilient means biasing said valve toward said one position with less force than the bias provided by the normal fluid pressure in said other conduit, :1 first check valve in said one conduit and a second check valve in said other conduit, each of said check valves being normally closed but adapted to open responsive to the pressure of fluid flowing from the source in its respective conduit, and means defining a passage between said source and said other conduit for the flow of fluid from the source at a rate less than the rate of flow from said source to said one or other conduit when said selector valve is in said one or other position, respectively.

' 2. In a system for supplying fluid from a pressurized source to the inlet of an hydraulically operated device, the combination comprising means defining two fluid conduits, a selector valve disposed between said conduits and said source and having one position wherein one of said conduits and said source are connected and another position wherein the other of said conduits and said source are connected, said selector valve being biased toward said one position by fluid pressure in said one conduit and biased toward said other position by fluid pressure in said other conduit, resilient means biasing said selector valve toward said one position with less force than the bias provided by the normal fluid pressure in said other. conduit, a first normally closed check valve disposed between said one conduit and said inlet and biased toward closed position by fluid pressure in said inlet, a second normally closed check valve disposed between said other conduit and said inlet and biased toward closed position by fluid pressure in said inlet, each of said check valves being adapted to open responsive to fluid pressure. in its respective conduit, means defining a passage between said source and said other conduit for flow of fluid from the source at a rate less than the rate of flow from the source to the said one or other conduit when said selector valve is in said one or other position, respectively, and a normally open bleed valve disposed between each of said conduits and said inlet for the flow of fluid from said inlet to said source in the absence of fluid pressure at said source, the bleed valve associated with said one conduit being adapted to close in the absence of fluid pressure in said one conduit and responsive to pressure in said inlet resulting from flow through said other conduit, and the bleed valve associated with said other conduit being adapted to .close in the absence of fluid pressure in said other conduit and responsive to pressure in said inlet resulting from flow through saidone conduit.

3. In a system for supplying fluid from a pressurized source to the inlet of an hydraulically operated device, the combination comprising means defining two fluid conduits connected with said device, a selector valve disposed between said conduits and said source and having one position wherein one of said conduits is connected with said source and another position wherein the other of said conduits is connected with said source, said valve being biased toward said one position by fluid pressure in said one conduit and biased toward said other position by fluid pressure in said other conduit, resilient means biasing said valve toward said one position with less force than the bias provided by the normal fluid pressure in said other conduit, a normally closed pressurizing valve disposed in said one conduit and adapted to open at a preselected pressure of flow therein from said source, and means defining a passage between said source and said other conduit for the flow of fluid from the source at a rate less than the rate of flow from the source to said one conduit when said selector valve is in said one position, the pressurizing valve serving to establish pressure in said other conduit to bias said selector valve to said other position if a break occurs in said one conduit between the pressurizng valve and the said hydraulically operated device when there is no substantial pressure in said other conduit.

4. In a system for supplying fluid from a pressurized source, the combination comprising means defining two fluid conduits, a selector valve disposed between said conduits and said source and having one position wherein one of said conduits is connected with said source and another position wherein the other of said conduits is connected with said source, said valve being biased toward said one position by fluid pressure in said one conduit and biased toward said other position by fluid pressure in said other conduit, resilient means biasing said valve toward said one position with less force than the bias provided by the normal fluid pressure in said other conduit, a normally closed pressurizing valve disposed in said one conduit adjacent said selector valve and adapted to open at a preselected pressure therein, a first check valve in said one conduit and a second check valve in said other conduit, each of said check valves being normally closed but adapted to open responsive to the pressure of fluid flowing from the source in its respective conduit, and means defining a passage between said source and said other conduit for the flow of fluid from the source at a rate less than the rate of flow from said source to said one or other conduit when said selector valve is in said one or other position, the pressurizing valve serving to establish pressure in said other conduit to bias said selector valve to said other position if a break occurs in said one conduit when there is no substantial pressure in said other conduit.

5. In a system for supplying fluid from a pressurized source to the inlet of an hydraulically operated device, the combination comprising means defining two fluid conduits, a selector valve disposed between said conduits and said source and having one position wherein one of said conduits is connected with said source and another position wherein the other of said conduits is connected with said source, said selector valve being biased toward said one position by fluid pressure in said one conduit and biased toward said other position by fluid pressure in said other conduit, resilient means biasing said selector valve toward said one position with less force than the bias provided by the normal fluid pressure in said other conduit, a normally closed pressurizing valve disposed in said one conduit and adapted to open at a preselected pressure therein, a first normally closed check valve disposed between said one conduit and said inlet and biased toward closed position by fluid pressure in said inlet, a second normally closed check valve disposed between said other conduit and said inlet and biased toward closed position by fluid pressure in said inlet, each of said check valves being adapted to open responsive to fluid pressure in its respective conduit, and means defining a passage between said source and said other conduit for the flow of fluid from the source at a rate less than the rate of flow from the source to said one conduit when said selector valve is in said one position, the pressurizing valve serving to establish pressure in said other conduit to bias said selector valve to said other position if a break occurs in said one conduit between the pressurizing valve and the said hydraulically operated device when there is no substantial pressure in said other conduit.

6. In a system for supplying fluid from a pressurized source to the inlet of an hydraulically operated device, the combination comprising means defining two fluid conduits, a selector valve disposed between said conduits and said source and having one position wherein one of said conduits is connected with said source and another position wherein the other of said conduits is connected with said source, said selector valve being biased toward said one position by fluid pressure in said one conduit and biased toward said other position by fluid pressure in said other conduit resilient means biasing said selector valve toward said one position with less force than the bias provided by the normal fluid pressure in said other conduit, a normally closed pressurizing valve disposed in said one conduit and adapted to open at a preselected pressure of flow therein from said source, means defining a passage around said pressurizing valve, valve means adapted to close said passage responsive to pressure of flow from said source and adapted to open to accommodate flow from said inlet in the absence of flow from said source, a first normally closed check valve disposed between said one conduit and said inlet and biased toward closed position by fluid pressure in said inlet, a second normally closed check valve disposed between said other conduit and said inlet and biased toward closed position by fluid pressure in said inlet, each of said check valves being adapted to open responsive to fluid pressure in its associated conduit, means defining a passage between said source and said other conduit for the flow of fluid from the source at a rate less than the rate of flow from the source to said one conduit when said selector valve is in said one position, and a normally open bleed valve disposed between each of said conduits and said inlet for the flow of fluid from said inlet to said source in the absence of fluid pressure at said source, the bleed valve associated with said one conduit being adapted to close in the absence of fluid pressure in said one conduit and responsive to pressure in said inlet resulting from flow through said other conduit, and the bleed valve associated with said other conduit being adapted to close in the absence of fluid pressure in said other conduit and responsive to pressure in said inlet resulting from flow through said one conduit.

7. In a system for supplying fluid from a pressurized source to the inlet of an hydraulically operated device, the combination comprising means defining two fluid conduits, a selector valve disposed between said conduits and said source and having one position wherein one of said conduits is connected with said source and another position wherein the other of said conduits is connected with said source, said selector valve being biased toward said one position by fluid pressure in said one conduit and biased toward said other position by fluid pressure in said other conduit, resilient means biasing said selector valve toward said one position with less force than the bias provided by the normal fluid pressure in said other conduit, a normally closed pressurizng valve disposed in said one conduit and adapted to open at a preselected pressure of flow therein from said source, means defining a passage around said pressurizing valve, valve means adapted to close said passage responsive to pressure of flow from said source and adapted to open to accommodate flow from said inlet in the absence of flow from said source, a first normally closed check valve 9 disposed between said one conduit and said inlet and biased toward closed position by fluid pressure in said inlet, a second normally closed check valve disposed between said other conduit and said inlet and biased toward closed position by fluid pressure in said inlet, each of said check valves being adapted to open responsive to fluid pressure in its associated conduit, means defining a passage between said source and said other conduit for the flow of fluid from the source at a rate less than the rate of flow from the source to said one conduit when said selector valve is in said one position, and a normally open bleed valve disposed in each of said check valves for the flow of fluid from said inlet to said source in the absence of fluid pressure at said source, the bleed valve associated with said first check valve being adapted to 10 close in the absence of fluid pressure in said one conduit and responsive to pressure in said inlet resulting from flow through said other conduit, and the bleed valve associated with said second check valve being adapted to close in the absence of fluid pressure in said other conduit and responsive to pressure in said inlet resulting from flow through said one conduit.

References Cited in the file of this patent UNITED STATES PATENTS 1,803,437 Riney et a1. May 5, 1931 2,604,900 Hewitt July 29, 1952 FOREIGN PATENTS 543,894 Great Britain Mar. 18, 1942 

